CHICAGO -- Caffeine is the most widely used stimulant in the world, consumed in coffee, tea and soft drinks by hundreds of millions of people to get started in the morning and as a pick-me-up during the day. That people like the jolt they get from caffeine is no secret, but what caffeine does in the brain has been unknown. Now a team of Austrian researchers using advanced brain imaging technology have discovered that caffeine makes people more alert by perking up part of the brain involved in short-term memory, the kind that helps focus attention on the tasks at hand. And Americans seem most in need of concentrating their thoughts since their average daily consumption of 236 milligrams of caffeine, equivalent to more than 4.5 cups of coffee, is three times the world average. "Almost all of us drink coffee or something with caffeine in it and we know why, because we want to be more awake or feel better," said Dr. Florian Koppelstaetter of the Medical University Innsbruck in Austria. "We wanted to know what effect one to two cups of coffee would have on short-term memory." Reporting Wednesday at the Radiological Society of North America meeting in Chicago, Koppelstaetter said that functional magnetic resonance imaging, or fMRI, was used to measure brain function in 15 healthy volunteers before and after consuming coffee. The findings revealed increased activity in the frontal lobe, where working memory is centered, and the anterior cingulum, which controls attention, in volunteers after consuming 100 milligrams of caffeine, the equivalent of about two cups of coffee. These areas showed no increased activity when the subjects drank the same fluid without caffeine in it. "The increased activity means you are more able to focus," Koppelstaetter said. Short-term memory lasts about 30 to 45 seconds and stores a small amount of information for a limited amount of time. It's the kind of memory used to look up a telephone number and remember it long enough to dial it. Long-term memory, on the other hand, stores an unlimited amount of information for an unlimited amount of time. In another report presented at the meeting, researchers from New York's Albert Einstein Medical School found that marijuana smoking may increase the risk of schizophrenia in people who have a genetic susceptibility to the disease.